[dtn-security] security layer

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Hello, 
When we design a secure system, the security should be provided at each layer to make a good secure system. 
If we develop a system on top of DTN based network, then which layer will be best suitable to provide security extensions.  

Suppose, we have underlying sensor network and on top DTN as overlay network, which layer we should provide the security services 
for such system.  I think, to get benefits like, generic solution, we should introduce security extensions to bundle layer ... 

                             Bundle Layer 
                              Security 
                             Transport Layer 
                             Network Layer
                             Data Link Layer 
                             Physical Layer  

Please comment on this and appropriate suggestions for building a security architecture for overlay network on top of sensor networks. 

Nasir